In recent years, electronic devices such as portable information devices and information home appliances are achieving higher functionality, with the progress of digital technology. With an increase in the functionality of such electronic devices, rapid progress has been made in miniaturization of semiconductor devices used therefor and increase in the speed thereof. Especially, the use application of nonvolatile memory with a large capacity as represented by flash memory has been rapidly expanded. Furthermore, development of a variable resistance semiconductor memory device (ReRAM) using a so-called variable resistance element is now in progress, as a new nonvolatile memory for the next generation that replaces the above flash memory.
Here, a variable resistance element is an element that has a property that a resistance value reversibly changes due to an electrical signal, and is further capable of storing information corresponding to this resistance value in a nonvolatile manner. The resistance value of a phase change element (PCRAM) changes due to a crystal condition being changed by heat generated by an electric stimulus. On the other hand, unlike the phase change element (PCRAM), the resistance value of a variable resistance element is changed due to a change between the oxidation and reduction states of variable resistance material, which is directly caused by an electric stimulus, that is, via a transfer of electron.
As an example of a semiconductor memory device with a large capacity including such a variable resistance element, a crosspoint semiconductor memory device is known. In the case of such crosspoint ReRAM, a diode is inserted in a nonvolatile memory element of each memory cell in series (e.g., see Patent Literature 1). Accordingly, when the resistance value of a selected nonvolatile memory element (memory cell) that is formed at a crosspoint at which a word line and a bit line are three-dimensionally intersected with each other is read, the influence of a current that flows through a memory element that is not selected (sneak current) can be avoided.
FIG. 10 shows a semiconductor memory device including conventional variable resistance elements. The semiconductor memory device shown in FIG. 10 is a crosspoint memory cell array having bit lines 210, word lines 220, and memory cells 280 each of which is formed at a crosspoint of the lines. Further, the memory cells 280 are each formed by connecting, in series, a variable resistance element 260 that stores information according to a change in the electrical resistance due to electrical stress, and a two-terminal diode 270 that allows a current to flow bidirectionally and has a nonlinear current-voltage characteristic. The bit lines 210 that are upper wiring are electrically connected with the diodes 270, and the word lines 220 that are lower wiring are electrically connected with the variable resistance elements 260. Since a current bidirectionally flows into the diodes 270 when the memory cells 280 are rewritten, an increase in the capacity can be achieved by using diodes (such as varistors) with a nonlinear current-voltage characteristic with respect to both polarity directions of applied voltage (both of the positive and negative voltage sides), for example.
A semiconductor memory device is also proposed in which a variable load resistor is connected to crosspoint ReRAM (e.g., see Patent Literature 2).
FIG. 11 is a block diagram showing a relationship among a variable resistance element of a selected memory cell, a load circuit, and a peripheral circuit of a conventional semiconductor memory device.
A high resistance state and a low resistance state of the semiconductor memory device shown in FIG. 11 can be stabilized by changing the resistance value of the load circuit, in the rewriting of data to a memory cell.